[adservice] add new adservice ff to demo docs (#4172)

Co-authored-by: Juliano Costa <julianocosta89@outlook.com>

content/en/docs/demo/feature-flags.md

@@ -16,6 +16,7 @@ values are stored in the `demo.flagd.json` file. To enable a flag, change the
 | Feature Flag                        | Service(s)       | Description                                                                                               |
 | ----------------------------------- | ---------------- | --------------------------------------------------------------------------------------------------------- |
 | `adServiceFailure`                  | Ad Service       | Generate an error for `GetAds` 1/10th of the time                                                         |
+| `adServiceManualGc`                 | Ad Service       | Trigger full manual garbage collections in the ad service                                                 |
 | `cartServiceFailure`                | Cart Service     | Generate an error for `EmptyCart` 1/10th of the time                                                      |
 | `productCatalogFailure`             | Product Catalog  | Generate an error for `GetProduct` requests with product id: `OLJCESPC7Z`                                 |
 | `recommendationServiceCacheFailure` | Recommendation   | Create a memory leak due to an exponentially growing cache. 1.4x growth, 50% of requests trigger growth.  |